A company which supplies Halal meat to prisons has had its contract suspended after traces of pork DNA were found in pies and pasties.
The Ministry of Justice said it had withdrawn the products immediately, while the Justice Minister Jeremy Wright said the Prison Service was investigating "as a matter of urgency".
It comes after traces of horsemeat were found in burgers sold by some UK supermarkets , including Tesco.
"All prisons have been informed about this very regrettable incident and we reported this issue to the Food Standards Agency immediately," a spokesman for the Ministry of Justice said.
"We are taking immediate steps to suspend the contract with the relevant sub-contractor."
Mr Wright said: "This is an absolutely unacceptable situation and one which we regret greatly.
"This must be distressing for those affected and they can be reassured we are doing everything we can to resolve the situation. The Prison Service is investigating this as a matter of urgency."
The company involved has not been named and it is not known where the firm is based.
Islamic law forbids the consumption of pork and prison rules dictate that Halal meat must be offered to inmates.
"The labelling system and adherence to the EU regulations of labelling must be stricter," said Masood Khawaja, from the Halal Food Authority.
"An enforcing agency like the Food Standards Agency should take action, it has happened many times."
Juliet Lyon, director of the Prison Reform Trust, said: "This lapse will have offended and distressed high numbers of Muslim prisoners and their families so apologising, suspending the supplier and investigating the incident are the right steps for the Ministry of Justice to take."
A spokesman for the Food Standards Agency said a local authority investigation would look into the cause of the contamination and determine where the products had been sent.
It is due to meet with retailers and suppliers to remind them of their responsibilities surrounding food labelling after the recent horsemeat scandal.
Burger King has dropped the Irish food processing plant which supplied the contaminated burgers.
The Silvercrest plant in County Monaghan, which is part of the ABP Food Group, has also lost contracts with Tesco, Aldi and the Co-operative Group.
A sample of Tesco's Everyday Value Beef Burgers was found to contain 29% horsemeat.